Pipe system couplings and unions that are used in fluid handling systems typically have a seal mechanism that depends on the use of a compressible elastomeric member to provide a liquid-tight seal. Such pipe couplings are used in the piping systems of high-purity manufacturing processes, such as semiconductor manufacturing, that depend on the integrity of such seals to both protect the transported process fluid against contamination from an external source, and to protect the immediate environment and personnel from possible contact with spilled process fluid. It is, therefore, desired that the seal integrity of pipe system couplings used in such applications be extremely high.
Pipe system couplings that depend on compressible elastomeric members such as O-rings typically consist of complementary first and second coupling members. The first coupling member includes a groove located in a facing surface to accommodate partial placement of an O-ring seal therein, and the second coupling member has a flat facing surface that contacts an opposite surface of the O-ring seal. The O-ring seal can be made of conventional elastomeric material such as rubber, nitrile rubber and the like. The O-ring seal is positioned between the coupling members concentrically around the flow path of the process fluid through the coupling members. A threaded coupling nut can be used to force the coupling members together and compress the O-ring seal therebetween. A liquid-tight seal can be achieved by applying a sufficient torque to the nut to compress the O-ring seal to completely fill the respective grooves.
One disadvantage of pipe system couplings that use an O-ring sealing mechanism relates to the placement of the O-ring seal between the coupling members and the degree of compression by the O-ring seal. When the O-ring seal is compressed between the coupling member, by applying sufficient torque to the nut, there remains facing surface portions of each coupling member between the fluid flow path and the O-ring seal that are wetted by the process fluid. This wetted area forms a hold-up volume for process fluid within the coupling when fluid flow is terminated, causing the process fluid to be retained therein.
The presence of a hold-up volume in a pipe system coupling is not desirable, as the retained process fluid could introduce contamination into the fluid handling system if a subsequent different process fluid is transferred therethrough. Additionally, when the retained process fluid is water, e.g., deionized water used in high-purity rinsing applications and the like, such water can stagnate and induce biological growth. Such Biological growth in high-purity piping systems is not desirable because it contaminates the process fluids and any components that have been manufactured using such process fluid.
Another disadvantage of pipe system couplings which use O-ring-type seal mechanisms relates to the eventual degradation of the O-ring material when it is placed in contact with corrosive or caustic process fluids. To perform their function properly, an O-ring must necessarily be made from a material capable of being elastomerically compressed, such as rubber, modified rubber and the like. Such materials are known to experience some degree of degradation when exposed to caustic or corrosive process fluids, such as those used in the semiconductor manufacturing industry. As O-ring seals degrade, the degraded material enters the process fluid, causing it to become contaminated, which in turn contaminates and components that are contacted by the fluid. Furthermore, once the O-ring begins to degrade the liquid-tight seal is compromised, resulting in leakage into or out from the system.
A proposed solution to the problem of degrading O-rings was to manufacture an O-ring seal having an elastomeric body coated with a chemically resistant fluoropolymeric compound. Such fluoropolymeric materials, for example, are tetrafluoroethylene sold by DuPont under the trademark Teflon.RTM.. In order to retain a sufficient degree of compressibility using the coated O-ring it is necessary that such Teflon.RTM. coatings on the O-ring be very thin. Process fluids are known to permeate through such thin Teflon.RTM. coatings, thereby exposing the easily degradable elastomeric material to the process fluid. Additionally, thin Teflon.RTM. coatings are known to degrade when exposed to caustic or corrosive process fluids such as hydrochloric acid and hydrofluoric acid, thereby also exposing the easily degradable elastomeric material. Furthermore, the use of Teflon.RTM. coated O-rings does not remedy the problem of creating a hold up volume and related process/product contamination.
It is, therefore, desired that a pipe system coupling or union be constructed having a sealing mechanism that does not contaminate the process fluid, and that does not have a hold-up volume. It is further desired that the pipe system coupling sealing mechanism be easy to use without the need to depend on specialized installation equipment, be formed from readily available materials, and be made using conventional manufacturing techniques.